The present invention relates to improvements in external liquid level gauges of the type illustrated and described in U.S. Pat. No. 3,696,675 to A. Scott Gilmour issued Oct. 10, 1972.
The '675 patent discloses an elongate strip of material coated with a substance which varies chromatically with variations in temperature. A layered strip is employed which comprises a base of "Mylar" having a backing of pressure sensitive adhesive and a layer of liquid crystal or other thermochromatic material coated on a base layer of polymeric film. The thermochromatic coated polymeric film layer is secured by an adhesive to the "Mylar" layer. A protective layer to prevent deterioration from ultra violet radiation is provided over the liquid crystal layer. In some presently known gauges of this type other thermochromatic materials, as for example, mercurous oxide are employed.
The contents level with this type of gauge is determined by sensing the temperature differential between the portion of the container wall above and below the surface of the contents. The container wall may be heated by application of hot water, or a cloth saturated with hot water or in any other suitable or desirable manner. Because the liquid absorbs heat at a rate different from that of the gas, air or free space there results a distinct difference in the temperature of the container wall above and below the surface of the liquid.
The thermochromatic material varies in color with variations in temperature. Thus a line of demarcation is produced at the surface of the liquid contents by the differing colors resulting from the temperature differential. It is thus apparent that good heat conduction between the container wall and the temperature sensing thermochromatic material is essential. To some extent the plastic base layer and the adhesive used in present liquid level gauges of the type disclosed in the '675 Patent serve as heat insulators. It would be advantageous to improve the heat conduction between the container wall and the sensing material. Improved heat conduction requires less applied heat and produce greater contrast at the potions above and below the contents level.
This type of gauge is widely used as propane tanks for backyard grills as well as heating units and cook stoves on recreational vehicles. The containers for propane come in different heights. Thus a different length strip is required for each container. In some instances the empty containers are exchanged for full containers rather than being refilled at the time of purchasing the propane. Since the gauge is secured by adhesive, removal results in destruction of the gauge; a new gauge must be attached each time an empty container is exchanged for a full one. A gauge that can be repeatedly removed and replaced or relocated would enable a single length strip to be utilized on containers of differing heights because the gauge on a tall container could be moved vertically downward as the tank is emptied. Furthermore it would be more economical to be able to transfer the gauge from one container to another when an empty container is exchanged for a full one. In addition, when using two or more containers, a single gauge could be used and transferred from container to container.